++
The problem of moving the body through water is fundamentally
not so different from that of moving it on land. As in walking,
it is necessary to push against something to move the body from one
place to another. The chief differences between locomotion in the
water and locomotion on land are that (1) in the water the body
is concerned with buoyancy rather than with the force of gravity,
(2) the substance against which it pushes affords less resistance
to the push, (3) the medium through which it moves affords more
resistance to the body, and (4) as a means of getting the greatest
benefit from the buoyancy and of reducing the resistance afforded
by the water, it is customary to maintain a horizontal, rather than
a vertical, position. (Review the discussion of buoyancy in Buoyancy.) The practical problem in swimming is not to keep
from sinking, as novices are inclined to believe, but to get the
mouth out of the water at rhythmic intervals in order to permit
regular breathing. This is a matter of coordination, not buoyancy.
++
In swimming, as in all motion, the initial mechanical problem
is to overcome the inertia of the body. Once the body is in motion,
the problem is to overcome the forces that tend to hinder it. In terrestrial
locomotion the body exerts its force against the supporting surface,
the ground, to overcome inertia. The forces resisting the progress
of the body are the forces of gravity and air resistance. In aquatic
locomotion the water is both the supporting medium and the source
of resistance. In swimming the hands and feet depend on the reaction
force of the water in order that the force may be transmitted to
the body. At the same time, the body must overcome the resistance
afforded by the water.
++
The speed obtained in swimming any stroke
depends on the stroke length and stroke frequency. The length
of the stroke is the result of the forces that move the swimmer
forward in reaction to the movements of the arms and legs, and of
the resistance of the water in the opposite direction. In the front
crawl the arms are the primary source of power, whereas in the breaststroke
the legs dominate. Regardless of the stroke, the actions of the
arms and legs appear to result in a combination of lift and drag
forces that then propel the body forward.
++
Four different types of water resistance act to decrease the
stroke length. Form drag, or pressure
drag, is the resistance that is due to the surface area of the front
of the body as it meets the oncoming water. This type of resistance
has the greatest effect on retarding the swimmer. Streamlining the
body by changing its position in the water decreases form drag. Surface drag is caused by the resistance
of the water next to the body. Although it has little effect on
forward progress, swimmers have been known to shave the hair from
their bodies as a means of decreasing skin friction. A third form
of drag is that which occurs at the surface as the body moves along, partially
in water and partially in air. The resultant waves form an additional
resistance to forward progress called wave
drag. The amount of this resistance depends on both the speed
and movements of the body as it progresses through the water. A
fourth but minor resistance is turbulence that forms behind the body,
causing it to pull some water along with it.
++
The frequency of the stroke depends
on the amount of time spent per stroke cycle. This, in turn, is
related to the nature of the stroke pattern and the muscle torques
of the arms and legs. Thus the major problem in the mechanics of
swimming is the minimization of the resistance that is due to the
water, which is either pushed out of the way or dragged along, and
the advantageous application of the force of the arms and legs.
The swimmer reduces resistance by streamlining the body position
through relaxing in the recovery phase of the stroke and by eliminating
useless motion and tensions. The propulsive force is increased with
improvements in technique and conditioning.
+++
Analysis of
the Sprint Crawl
++
As an example of aquatic locomotion, the sprint crawl (Figure
20.2) has been chosen for analysis. The position of the head and
trunk and the movement of the head in breathing are described briefly.
The arm and leg strokes are described in somewhat greater detail,
and their propulsive phases are analyzed anatomically.
++
++
The head and trunk have three important functions in swimming,
particularly in speed swimming. These are minimizing resistance,
enabling the swimmer to breathe, and providing a stable anchorage
for the arm and leg muscles to effect a maximum propulsive force.
The position of the body is the key to reducing resistance. The
body is as horizontal as possible, with the feet below the surface
and the head breaking the water at hairline level. The flatter the
body, the less drag there will be to decrease the swimmer’s
speed. The exact position of the body varies with the anatomical build
and buoyancy of the individual, as well as with the speed of the
stroke. The greater the buoyancy and speed, the higher the body
will ride in the water. A common mistake is to lift the head too
much. If the head is held too high or tipped back too far, it makes
the swimmer’s legs drop, causing a broader frontal surface
and therefore more resistance. By static contraction of the rectus
abdominis, the spine is held in a position of slight flexion—or
at least of incomplete extension—and the pelvis in a position
of slightly decreased inclination.
++
Lateral movements of the trunk will also increase the resistance
to forward movements and should be minimized. Any circular movement
of the arms or legs causes a countermovement in the rest of the
body. A wide swing of the arms on recovery produces a lateral, opposite
fishtail action of the legs. Lateral flexion of the head and neck
also results in a counteraction. The turning of the head for inhaling
must be accomplished with the least possible interference with the
rhythm of the arm and leg action and with the progress of the body
through the water. It is essential not to lift the head for breathing
but rather to rotate it on its longitudinal axis while tucking the
chin in close to the side of the neck. In this position the face
appears to be resting on the bow wave, and the mouth is just above
the surface of the water. After a quick inhalation, the face is
again turned forward with the eyes in the horizontal plane and the
nose and chin in the midsagittal plane of the body. Although breathing
with every arm cycle is preferable in distance events, sprints are
better swum with fewer breathing cycles because the turning of the
head, even when done properly, causes additional resistance.
++
To provide a firm base of attachment for the muscles of the arms
and thighs, the trunk must be held steady. By the alternating action
of the left and right oblique abdominals and spinal extensors, the
spine and pelvis are stabilized against the pull of the shoulder
and hip muscles. Thus they permit the latter to exert all their
force on the limbs for the propulsive movements.
++
Because the arm stroke provides approximately 85 percent of the
total power, the entry of the arm into the water should place it
in the most advantageous position for exerting force that will be effective
in driving the body forward. Its position on entry is with the forearm
high and the elbow pointing to the side. The hand passes in front
of the shoulder in preparation for the entry and then, reaching
forward, it is driven forward and downward into the water directly
in front of the shoulder. The elbow is slightly flexed at the beginning
of the hand entry but extends during the entry. The brief moment
between entry and the beginning of the chief propulsive action is
known as the support phase, and its purpose is to keep the head
and shoulders high in the water. The pressure of the forearm and
hand is mostly downward and then backward, thus producing an upward
and forward reactive force (Figure 20.2a, b).
+++
Catch, Pull,
and Push
++
The moment at which the chief propulsive action changes from
downward to backward constitutes the catch. The pull phase begins
with the first backward movement of the hand. This occurs when the
hand is 5 to 10 inches below the surface and involves a quick inward
movement of the hand and arm that serves to bring the hand to a
position in front of the axis of the body in such a way that the
body weight is balanced above the arm. The upper arm is approximately
vertical, a position that favors the large muscles (sternal portion
of the pectoralis major and latissimus dorsi) for their task of
pulling the arm downward and backward. Because the purpose of the
stroke is to drive the body forward, it is essential to apply maximum
force over the longest possible distance. This is best done by keeping
the elbow high during the first part of the pull and by bending
the elbow as the arm is pulled under the body (Figure 20.2c, d).
The maximum bend occurs halfway through the pull, when the hand
begins to push the water backward.
++
This elbow action assists in producing the S-curve, which allows
for the creation of propulsive lift. Flexion at the elbow also reduces
the moment arm of the upper extremity, reducing resistance to motion.
In the pull phase the feel is as if the hand is being pushed backward
through the water. In actuality, lift and drag act to stabilize
the hand in the water. The body is then pulled forward over the
relatively stationary hand.
++
The transition from pull to push occurs as the arm passes under
the shoulder. The upper arm remains nearly vertical as the forearm
gradually extends until it is in front of the hip, at which time
the upper arm extends and the hand gives a quick push backward (Figure
20.2e, f). It would seem that the ideal direction of the swimmer’s
force should be directly backward for maximum forward horizontal
movement. In actuality the path of the hand of most good swimmers
is more like an inverted question mark. This apparent sculling motion
constantly varies the angle between the surface of the hand and
the direction of hand motion (angle of attack). The combination
of drag forces and lift forces (see Bernouilli’s principle in
Lift and Drag) that will be utilized for propulsion is extremely dependent
on this angle of attack. By varying the angle of attack, the optimum
lift and drag resultant for every position of the hand can be attained.
This means that as the hand velocity direction shifts with the curvilinear
path of the hand, the resultant of the drag and lift forces can still
be maintained in line with the body’s line of motion (Toussaint
and Beck 1992).
+++
Brief Anatomical
Analysis of Propulsive Phase of Arm Stroke
++
Front crawl swimming is an excellent total body exercise. It
has been estimated that at least 44 muscles act to produce the movements
of the crawl. If all active muscles (antagonists, synergists, etc.)
are considered, the estimate rises to 170 single muscles (Clarys
and Cabri 1993). For this reason, only the most active prime movers
will be covered in this analysis (Table 20.1).
++
++
The elbow is now near the surface with the hand slightly lower
and posterior to it and the palm facing mostly upward. The pressure
of the forearm and hand now being relaxed, the elbow and shoulder
are raised until the hand is out of the water. The elbow leaves
the water first and swings forward and upward with the hand trailing
behind it, moving from a position near the hip to a position in
front of the shoulder, preparatory to a new entry (Figure 20.2g,
h). The movement of the arm from release to the completion of recovery
is continuous. It is important that no break occur because this
would mean a loss of momentum and would necessitate an additional
force for overcoming inertia, or at least for regaining the lost
velocity.
++
As the elbow is brought forward, it remains above the level of
the hand throughout the recovery and entry, with the forearm virtually
horizontal as the arm moves forward past the shoulder and the hand
in line with the forearm. Finally, as the hand passes the head,
the arm reaches forward in preparation for the entry, the shoulder
girdle remains high, the tip of the elbow is above shoulder level
pointing to the side, and the forearm then points downward from
the elbow with the wrist slightly flexed, the palm facing the water,
and the fingers aiming forward and downward into the water. Individual
differences in style in the arm recovery are likely to occur. These
variations are most likely due to differences in shoulder range
of motion. Unless such variations interfere with other aspects of
the stroke, they are most likely insignificant (Table 20.2).
++
++
The leg stroke most often used in the sprint crawl is the flutter
kick. Whether or not it contributes to the propulsive force has
been questioned. It is generally acknowledged that the primary role
for the kick is that of a stabilizer and neutralizer, and that therefore
its timing with respect to the arm’s action is critical.
In this stroke the legs are relatively close together as they alternate
in an up and down movement, with the feet attaining a maximum stride
of about 1 to 2 feet. The width of the kick depends on such factors
as the swimmer’s build and strength and the speed of the stroke.
In both the upstroke and downstroke, the movement, described as
whip-like or lashing, starts at the hip joint and progresses through
the knees to the ankle and feet. Unlike the arms, whose movements
alternate between propulsion and recovery, both phases of the leg
stroke are propulsive, if anything. Flexibility in the ankles is
important in the kick, and those with a greater range of plantar
flexion have an advantage. In the downstroke the thrust is downward
and backward and, in the upstroke, upward and backward.
++
The downstroke begins with a downward drive of the thigh. The
thigh flexes only slightly, and the knee, which was in a position
of flexion at the completion of the upstroke, extends completely
by the end of the downward movement. The ankle and foot remain in
plantar flexion, probably being held in this position by the pressure
of the water against the dorsal surface of the foot (Figure 20.2a,
b). It seems likely that the dorsiflexors contract statically to
stabilize the foot against this pressure. Throughout the downstroke
the foot remains in a slight toeing-in position. The heels should
not be allowed to drift apart in an attempt to facilitate the intoeing,
because this would involve rotation of the thigh and would cut down
on the driving power of the limb, as well as increase the form drag
(Table 20.3).
++
++
At the completion of the downstroke the thigh is in a position
of slight flexion, the knee is completely extended, and the ankle
is incompletely plantar flexed. The upstroke begins with thigh extension.
Slight knee flexion develops near the end of the stroke at the same
time the opposite leg is finishing the downstroke. The movements
of the three major segments of the lower extremity are forceful
in the upstroke but are under such good control that the foot stops
just below the surface of the water. To break through the surface
constitutes a major error, as it causes an immediate reduction in
propulsive force (Figure 20.2d–f; Table 20.4).
++
++
The front crawl consists of a six-beat leg kick for every complete
arm cycle. This would imply that three leg beats occur for each
arm. This uneven number of leg motions on each arm stroke act to
maintain the body in balance around the vertical axis while still
allowing for the body roll necessary for breathing. Breathing occurs
on one side of the body as the arm is lifted from the water. The
head is turned with the chin tucked toward the axilla. The breath
should be only an intake of air. Exhalation occurs underwater.
++
The arms would seem to be cycling in 180-degree opposition to
each other. As one arm is in the entry and catch phase of the stroke,
the opposite arm will be in release and recovery. In fact, this is
true in a coordination currently referred to as opposition. As swim
velocity increases, however, it is likely that the arm cycles will
move closer together so that as one arm finishes the pull and moves
to the push, the opposite arm is beginning the entry and catch phase.
This coordination puts both hands in the water simultaneously for
a brief instant. In the recreational swimmer, opposition coordination
is the most common (Chollet et al. 2000; Seifert et al. 2005).
++
Other factors of importance to the crawl stroke swimmer and to
the coach are the rhythm of the stroke as a whole, the relaxation
of the body, and the flexibility of the joints, particularly of
the shoulders and ankles. Of these, possibly the last named is of
greatest interest to the kinesiologist. The serious swimmer will
want to know how to increase the range of motion in shoulder joints and
ankles—that is, how to stretch the pectorals and anterior
ligaments of the shoulders and how to gain greater plantar flexion
of the feet. The kinesiology student should be able to originate
several exercises that would accomplish this. One method that is
frequently used is dry land exercise using various forms of elastic
resistance.
+++
Sample Analysis
of a Common Fault in the Crawl Stroke
++
A rigid flutter kick is a common
fault of beginners learning the standard crawl stroke. It is included
in this text as an example of how the kinesiologist can analyze
a common fault and use the analysis as a basis for making constructive
suggestions in teaching.
++
In the rigid flutter kick the movement is one of alternate flexion
and extension of the entire lower extremity, with the movement confined
to the hip joint instead of being transmitted successively through
the thigh to the knee joint and then through the leg to the ankle
and foot. The knee joints are fully extended throughout the kick,
and the feet and ankles are held in an unchanging position of plantar
flexion, the exact degree of this flexion varying with individuals.
This results in a narrower kick. A rigid flutter kick is obviously
less efficient than the correct kick. In brief, the rigid flutter
kick deviates from the correct form in that there is an absence
of knee and ankle flexion, an absence of relaxation at the end of
the downkick or beginning of the upkick, and an absence of fishtail
action of the sole of the foot against the water.
++
In the correct downkick the upward pressure of the water against
the lower leg causes flexion at the knee. In the rigid kick, however,
this is prevented by the tension of the quadriceps extensors. Normally,
the slight flexion at the knee is followed by extension during the
course of the downstroke but, when the knee is already rigidly extended,
this extension cannot take place. Similarly, the reduction of the
plantar flexion that should take place at the end of the downstroke
fails to occur because of the continuous contraction of the plantar
flexor muscles (soleus, peroneus longus and brevis, tibialis posterior,
flexor digitorum longus, and flexor hallucis longus).
++
In the upstroke the tension of the quadriceps extensors again
prevents the slight knee flexion that occurs when the kick is correctly
performed (see Figure 20.2). Throughout the stroke the extensors
of the lower back and the abdominal muscles contract to stabilize
the pelvis against the pull of the hip flexors and extensors. Normally
they relax momentarily just before the legs reverse their direction.
The tension in the muscles of the lower extremities spreads to these,
however, and the excess tension of these muscles causes interference
with the action of the diaphragm. This, in turn, results in less
efficient breathing and is an additional factor in causing fatigue.
++
The propulsive component of force that drives the body forward
is that which pushes the water directly backward. In the downstroke
this is provided most effectively by the instep of the foot, and
in the upstroke, by the sole. The amount of propulsive force developed
depends on the angle at which the instep and the sole of the foot
are held with respect to the surface of the water. In the upstroke
the best angle for the sole of the foot is possible only when the
knee is flexed. In the rigid kick the knee is straight, and the
sole of the foot is therefore not in the best position for providing propulsive
force.
++
In the correct form each limb acts as a series of levers—thigh,
lower leg, and foot—but in the rigid kick each limb acts
as one long lever with the force arm extending from the distal attachments
of the hip flexors and extensors to the axis of the hip joint. The
resistance arm consists of the entire length of the lever from the
instep or from the sole of the foot to the hip joint. The force acting
on this lever comes solely from the muscles of the hip joints. The
muscles of the knee and ankle do not contribute to the motion of
this lever, but when the limb is used as a series of levers, they
provide additional force. Inertia must be overcome with each reversal
of direction in the kick. Because, in the rigid flutter kick, the
stroke is shorter and faster than it should be, the muscles of the
hip joint, which have the double task of overcoming both the inertia
of the limb and the resistance of the water, are overburdened. They
must work harder and faster to meet the demands made on them by
the frequent changes of direction and the increased resistance of
the water due to the speed of movement. Ordinarily the upstroke
has an advantage over the downstroke because, when the stroke is
performed correctly, the sole of the foot is in a better position
to push back against the water than is the instep on the downstroke.
In the rigid kick this advantage is lost.
++
The rigid crawl flutter kick is associated with undue tension
of the quadriceps extensors at the knee joint, and of the plantar
flexors at the ankle joint during the changes of direction in both
the downkick and the upkick. There may also be unnecessary tension
of the abdominal muscles and the spinal extensors. Because the kick
is inefficient, it is carried on at a faster rate and through a narrower
arc than would be the correct kick for the individual swimmer. Conversely,
a rapid, narrow kick tends to be a rigid kick. These factors aid
in its recognition. In teaching the kick, it would seem the best
procedure is to insist on a slow, deep kick in the student’s
first attempts, and to increase the rhythm gradually to the desired
rate. Motivation should not be directed toward speed in the performance
of the “kick glide” in the teaching progression.
Also, one should not emphasize that the legs be held straight at
the knees. The emphasis should be put on the increased action at
the hips and at the ankles.